From the Super Mario Wiki
A Hidden Block is a block that contains coins, a star or random items in the Mario Party series. Its appearance changes with each game, sometimes resembling Brick Blocks, other times looking more like a small Treasure Chest. They only appear after landing on a space. It can give out anywhere from five to twenty coins or give out items such as Skeleton Keys, or even Stars. Mario Party DS features a one-time use item called a Block Sensor that makes a Hidden Block appear on the space the player lands on after using it.